The present invention relates to an infrared light source incorporating a semiconductor laser associated with mode selection and power control means. It is used in the production of a 1.55 .mu.m source for long-distance optical links.
Optical fibres functioning at 1.55 .mu.m and designed for long-distance optical telecommunications have a very limited attenuation (0.3 to 0.5 dB/km). but have a considerable wavelength dispersion (0.15 ps/nm/km). For long-distance links, e.g. 50 km, it is consequently necessary to use very narrow-spectrum light sources of a few tenths of a nanometer.
However, conventional semiconductor lasers guided by the gain or index have spectral widths of a few nanometers and this width increases further in the case of modulation. In order to reduce this width, use is made of a mode selection procedure, also called optical feedback, enabling the source to be made monofrequency. The principle of this optical feedback is illustrated in FIG. 1.
The laser 1 shown in FIG. 1a is constituted by a Perot-Fabry resonator, whose resonant frequencies are separated by .DELTA..lambda..sub.1, differing slightly from the quantity: (.lambda..sup.2 /2nL) in which .lambda. is the wavelength of the source, L the length of the Perot-Fabry resonator and n the effective index of the material.
The gain curve 2 of the amplifying medium selects certain of its frequencies. These frequencies define the longitudinal modes of the resonator (FIG. 1b). A laser is monochromatic or longitudinal monomodal, when only one of its modes is present (generally the highest energy mode).
The addition of an external mirror (FIG. 1a) in the vicinity of the laser and at a distance l of approximately equal to the length L thereof, with a curvature adopting the shape of the wave front of the radiation emitted by the laser, modifies the resonant state of the laser. The resonator constituted by this spherical mirror and the most remote plane mirror has a length L+l. This laser has resonant frequencies (FIG. 1c) spaced by .DELTA..lambda..sub.2, whose value is close to ##EQU1##
The coincidences of the resonances of the main resonator of length L and the auxiliary cavity of length L+l makes it possible, by adjustment of the position of mirror 3, to select one of the modes of the main cavity (FIG. 1d).
For example, for a 200-.mu.m-long laser whose mean effective index is 3.5, the separation between longitudinal modes is 16 .ANG., i.e. a frequency shift of 214 GHz.
Thus, apart from the mode selection produced by adding mirror 3, there is a reduction in the fineness of the emitted line. This is proportional to the quality factor of the Perot-Fabry resonator, which is itself linked with the reflection coefficient of the mirrors and the length of the resonator. For high reflection coefficients, the width of the resonance peaks of the resonator is approximately equal to: ##EQU2## in which c is the speed of light and r the reflection coefficient of the mirrors. This width is approximately 100 GHz for the laser referred to hereinbefore.
By increasing the reflection coefficient r by adding an external mirror, it is thus possible to reduce this width in proportion to the length of the optical path. Using an external mirror spaced from the main resonator by a length equal to that of the laser and with a reflection coefficient of the external mirror equal to 80%, the width of the line is substantially reduced and drops to a few GHz.
This mode selection of optical feedback procedure applied to semiconductor lasers is more particularly described in the article by K. R. Preston entitled "Simple spectral control technique for external cavity laser transmitters" published in "Electronics Letters" of 9th Dec. 1982, vol. 18, no. 25, as well as in the article by R. Ries and F. Sporleder entitled "Low frequency instabilities of laser diodes with optical feedback" published in the report of the European Conference on Optical Communications, (ECOC), held in Cannes on Sept. 21-24, 1982.
It is also known that semiconductor lasers can be made power dependent by a current feedback. By measuring the lighting power emitted by the laser, it is possible to form an electrical signal able to control the supply circuit of the laser, so that the power remains constant. Such a procedure is, for example, described in U.S. Pat. No. 4,293,827, granted on Oct. 6, 1981 to D. R. Scifres and R. D. Burnham entitled "Hybrid semiconductor laser detectors. In such a system, a photodetector receives the light emitted by the split rear face of the laser resonator, the front face allowing the useful beam to pass. The photodetector supplies an amplifier, which controls a current source, which supplies the semiconductor element of the laser. Thus, a control loop is formed.